Sputtering using an unbalanced magnetron

ABSTRACT

A sputtering process and magnetron especially advantageous for low-pressure plasma sputtering or sustained self-sputtering, in which the magnetron has a reduced area but full target coverage. The magnetron includes an outer pole face surrounding an inner pole face with a gap therebetween. The outer pole of the magnetron of the invention is smaller than that of a circular magnetron similarly extending from the center to the periphery of the target and has a substantially larger total magnetic intensity. Thereby, sputtering at low pressure and high ionization fraction is enabled.

RELATED APPLICATIONS

This application is a division of Ser. No. 09/918,136, filed Jul. 30,2001, to be issued on Sep. 14, 2004 as U.S. Pat. No. 6,790,323, which isa division of Ser. No. 09/546,798, filed Apr. 11, 2001, now issued asU.S. Pat. No. 6,306,265, which is a continuation in part of Ser. No.09/373,097, filed Aug. 12, 1999, now issued as U.S. Pat. No. 6,183,614,which is a continuation in part of Ser. No. 09/249,468, filed Feb. 12,1999, all of which are incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The invention relates generally to sputtering of materials. Inparticular, the invention relates to the magnetron creating a magneticfield to enhance sputtering and the resultant sputtering methods.

BACKGROUND ART

Sputtering, alternatively called physical vapor deposition (PVD), is themost prevalent method of depositing layers of metals and relatedmaterials in the fabrication of semiconductor integrated circuits. Aconventional PVD reactor 10 is illustrated schematically in crosssection in FIG. 1, and the illustration is based upon the Endura PVDReactor available from Applied Materials, Inc. of Santa Clara, Calif.The reactor 10 includes a vacuum chamber 12 sealed to a PVD target 14composed of the material, usually a metal, to be sputter deposited on awafer 16 held on a heater pedestal 18. A shield 20 held within thechamber protects the chamber wall 12 from the sputtered material andprovides the anode grounding plane. A selectable DC power supply 22negatively biases the target 14 to about −600VDC with respect to theshield 20. Conventionally, the pedestal 18 and hence the wafer 16 areleft electrically floating.

A gas source 24 supplies a sputtering working gas, typically thechemically inactive gas argon, to the chamber 12 through a mass flowcontroller 26. In reactive metallic nitride sputtering, for example, oftitanium nitride, nitrogen is supplied from another gas source 27through its own mass flow controller 26. Oxygen can also be supplied toproduce oxides such as Al₂O₃. The gases can be admitted to the top ofthe chamber, as illustrated, or at its bottom, either with one or moreinlet pipes penetrating the bottom of the shield or through the gapbetween the shield 20 and the pedestal 18. A vacuum system 28 maintainsthe chamber at a low pressure. Although the base pressure can be held toabout 10⁻⁷ Torr or even lower, the pressure of the working gas istypically maintained at between about 1 and 1000 mTorr. A computer-basedcontroller 30 controls the reactor including the DC power supply 22 andthe mass flow controllers 26.

When the argon is admitted into the chamber, the DC voltage between thetarget 14 and the shield 20 ignites the argon into a plasma, and thepositively charged argon ions are attracted to the negatively chargedtarget 14. The ions strike the target 14 at a substantial energy andcause target atoms or atomic clusters to be sputtered from the target14. Some of the target particles strike the wafer 16 and are therebydeposited on it, thereby forming a film of the target material. Inreactive sputtering of a metallic nitride, nitrogen is additionallyadmitted into the chamber 12, and it reacts with the sputtered metallicatoms to form a metallic nitride on the wafer 16.

To provide efficient sputtering, a magnetron 32 is positioned in back ofthe target 14. It has opposed magnets 34, 36 creating a magnetic fieldwithin the chamber in the neighborhood of the magnets 34, 36. Themagnetic field traps electrons and, for charge neutrality, the iondensity also increases to form a high-density plasma region 38 withinthe chamber adjacent to the magnetron 32. The magnetron 32 is usuallyrotated about the center of the target 14 to achieve full coverage insputtering of the target 14. The form of the magnetron is a subject ofthis patent application, and the illustrated form is intended to be onlysuggestive.

The advancing level of integration in semiconductor integrated circuitshas placed increasing demands upon sputtering equipment and processes.Many of the problems are associated with contact and via holes. Asillustrated in the cross-sectional view of FIG. 2, via or contact holes40 are etched through an interlevel dielectric layer 42 to reach aconductive feature 44 in the underlying layer or substrate 46.Sputtering is then used to fill metal into the hole 40 to provideinter-level electrical connections. If the underlying layer 46 is thesemiconductor substrate, the filled hole 40 is called a contact; if theunderlying layer is a lower-level metallization level, the filled hole40 is called a via. For simplicity, we will refer hereafter only tovias. The widths of inter-level vias have decreased to the neighborhoodof 0.25 μm and below while the thickness of the inter-level dielectrichas remained nearly constant at around 0.71 μm. As a result, the viaholes in advanced integrated circuits have increased aspect ratios ofthree and greater. For some technologies under development, aspectratios of six and even greater are required.

Such high aspect ratios present a problem for sputtering because mostforms of sputtering are not strongly anisotropic, a cosine dependenceoff the vertical being typical, so that the initially sputtered materialpreferentially deposits at the top of the hole and may bridge it, thuspreventing the filling of the bottom of the hole and creating a void inthe via metal.

It has become known, however, that deep hole filling can be facilitatedby causing a significant fraction of the sputtered particles to beionized in the plasma between the target 14 and the pedestal 18. Thepedestal 18 of FIG. 1, even if it is left electrically floating,develops a DC self-bias, which attracts ionized sputtered particles fromthe plasma across the plasma sheath adjacent to the pedestal 18 and deepinto the hole 40 in the dielectric layer 42. The effect can beaccentuated with additional DC or RF biasing of the pedestal electrode18 to additionally accelerate the ionized particles extracted across theplasma sheath towards the wafer 16, thereby controlling thedirectionality of sputter deposition. The process of sputtering with asignificant fraction of ionized sputtered atoms is called ionized metaldeposition or ionized metal plating (IMP). Two related quantitativemeasures of the effectiveness of hole filling are bottom coverage andside coverage. As illustrated schematically in FIG. 2, the initial phaseof sputtering deposits a layer 50, which has a surface or blanketthickness of s₁, a bottom thickness of s₂, and a sidewall thickness ofs₃. The bottom coverage is equal to s₂/s₁, and the sidewall coverage isequal to s₃/s₁. The model is overly simplified but in many situations isadequate.

One method of increasing the ionization fraction is to create ahigh-density plasma (HDP), such as by adding an RF coil around the sidesof the chamber 12 of FIG. 1. An HDP reactor not only creates ahigh-density argon plasma but also increases the ionization fraction ofthe sputtered atoms. However, HDP PVD reactors are new and relativelyexpensive, and the quality of the deposited films is not always thebest. It is desired to continue using the principally DC sputtering ofthe PVD reactor of FIG. 1.

Another method for increasing the ionization ratio is to use ahollow-cathode magnetron in which the target has the shape of a top hat.This type of reactor, though, runs very hot and the complexly shapedtargets are very expensive.

It has been observed that copper sputtered with either an inductivelycoupled HDP sputter reactor or a hollow-cathode reactor tends to form anundulatory copper film on the via sidewall, and further the depositedmetal tends to dewet. The variable thickness is particularly seriouswhen the sputtered copper layer is being used as a seed layer of apredetermined minimum thickness for a subsequent deposition process suchas electroplating to complete the copper hole filling.

A further problem in the prior art is that the sidewall coverage tendsto be asymmetric with the side facing the center of the target beingmore heavily coated than the more shielded side facing a larger solidangle outside the target. Not only does the asymmetry require excessivedeposition to achieve a seed layer of predetermined minimum thickness,it causes cross-shaped trenches used as alignment indicia in thephotolithography to appear to move as the trenches are asymmetricallynarrowed.

Another operational control that promotes deep hole filling is chamberpressure. It is generally believed that lower chamber pressures promotehole filling. At higher pressures, there is a higher probability thatsputtered particles, whether neutral or ionized, will collide with atomsof the argon carrier gas. Collisions tend to neutralize ions and torandomize velocities, both effects degrading hole filling. However, asdescribed before, the sputtering relies upon the existence of a plasmaat least adjacent to the target. If the pressure is reduced too much,the plasma collapses, although the minimum pressure is dependent uponseveral factors.

The extreme of low-pressure plasma sputtering is sustainedself-sputtering (SSS), as disclosed by Fu et al. in U.S. patentapplication Ser. No. 08/854,008, filed May 8, 1997. In SSS, the densityof positively ionized sputtered atoms is so high that a sufficientnumber are attracted back to the negatively biased target to resputtermore ionized atoms. Under the right conditions for a limited number oftarget metals, the self-sputtering sustains the plasma, and no argonworking gas is required. Copper is the metal most prone to SSS, but onlyunder conditions of high power and high magnetic field. Coppersputtering is being seriously developed because of copper's lowresistivity and low susceptibility to electromigration. However, forcopper SSS to become commercially feasible, a full-coverage, high-fieldmagnetron needs to be developed.

Increased power applied to the target allows reduced pressure, perhapsto the point of sustained self-sputtering. The increased power alsoincreases the ionization density. However, excessive power requiresexpensive power supplies and increased cooling. Power levels in excessof 30 kW are expensive and should be avoided if possible. In fact, thepertinent factor is not power but the power density in the area belowthe magnetron since that is the area of the high-density plasmapromoting effective sputtering. Hence, a small, high-field magnet wouldmost easily produce a high ionization density. For this reason, someprior art discloses a small circularly shaped magnet. However, such amagnetron requires not only rotation about the center of the target toprovide uniformity, but it also requires radial scanning to assure fulland fairly uniform coverage of the target. If full magnetron coverage isnot achieved, not only is the target not efficiently used, but moreimportantly the uniformity of sputter deposition is degraded, and someof the sputtered material redeposits on the target in areas that are notbeing sputtered. Furthermore, the material redeposited on unsputteredareas may build up to such a thickness that it is prone to flake off,producing severe particle problems. While radial scanning canpotentially avoid these problems, the required scanning mechanisms arecomplex and generally considered infeasible in a production environment.

One type of commercially available magnetron is kidney-shaped, asexemplified by Tepman in U.S. Pat. No. 5,320,728. Parker discloses moreexaggerated forms of this shape in U.S. Pat. No. 5,242,566. Asillustrated in plan view in FIG. 3, the Tepman magnetron 52 is based ona kidney shape for the magnetically opposed pole faces 54, 56 separatedby a circuitous gap 57 of nearly constant width. The pole faces 54, 56are magnetically coupled by unillustrated horseshoe magnets bridging thegap 57. The magnetron rotates about a rotational axis 58 at the centerof the target 14 and near the concave edge of the kidney-shaped innerpole face 54. The convexly curved outer periphery of the outer pole face56, which is generally parallel to the gap 57 in that area, is close tothe outer periphery of the usable portion if the target 14. This shapehas been optimized for high field and for uniform sputtering but has anarea that is nearly half that of the target. It is noted that themagnetic field is relatively weak in areas separated from the pole gap57.

For these reasons, it is desirable to develop a small, high-fieldmagnetron providing full coverage so as to promote deep hole filling andsustained copper self-sputtering.

SUMMARY OF THE INVENTION

The invention includes a sputtering magnetron having an oval or relatedshape of smaller area than a circle of equal diameter where the twodiameters extend along the target radius with respect to the typicalrotation axis of the magnetron. The shapes include racetracks, ellipses,egg shapes, triangles, and arced triangles asymmetrically positionedabout the target center.

The magnetron is rotated on the backside of the target about a pointpreferably near the magnetron's thin end, and the thicker end ispositioned more closely to the target periphery. Preferably, the totalmagnetic flux is greater outside than inside the half radius of thetarget.

The magnetic intensity away from the target can be increased for atriangular magnetron having a relatively small apex angle by using barmagnets.

The small area allows an electrical power density of at least 600 W/cm²to be applied from an 18 kW power supply to a fully covered sputteringtarget used to sputter deposit a 200 mm wafer.

The high power density and the magnetic field extending far away fromthe target are two means possible to produce a plasma wave which canfurther drive the plasma to a higher density and ionization.Advantageously, a primary plasma wave is generated at a higher frequencyin the range of hundreds of megahertz, which is parametrically convertedto another wave at a much lower frequency, for example, 5 to 75 MHz,corresponding to the thermal velocity of electrons in the plasmaproduced by capacitively coupling DC power to the target.

The magnetron is configured to produce less magnetic flux in its innerpole than in its surrounding outer pole. Thereby, the magnetic fieldreaches further into the sputtering chamber to promote low-pressuresputtering and sustained self-sputtering.

The invention also includes sputtering methods achievable with such amagnetron. The high magnetic field extending over a small closed areafacilitates sustained self-sputtering. Many metals not subject tosustained self-sputtering can be sputtered at chamber pressures of lessthan 0.5 milliTorr, often less than 0.2 milliTorr, and even at 0.1milliTorr. The bottom coverage can be further improved by applying an RFbias of less than 250 W to a pedestal electrode sized to support a 200mm wafer. Copper can be sputtered with 18 kW of DC power for a 330 mmtarget and 200 mm wafer either in a fully self-sustained mode or with aminimal chamber pressure of 0.3 milliTorr or less.

The invention provides for high-power density sputtering with powersupplies of reduced capacity.

The invention also includes sputtering under conditions, such as asufficiently high target power and high magnetic field away from thetarget, that a non-linear wave-beam interaction occurs that pumps energyinto plasma electrons, thereby increasing the plasma density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a DC plasma sputtering reactor.

FIG. 2 is a cross-sectional view of a inter-level via in a semiconductorintegrated circuit.

FIG. 3 is a plan view of a conventional magnetron.

FIG. 4 is a plan view of the pole pieces of an embodiment of themagnetron of the invention taken along the view line 4-4 of FIG. 7.

FIG. 5 is a plan view of the magnets used in the magnetron of FIG. 4.

FIG. 6 is a cross-sectional view of one of the magnets used inconjunction with the embodiments of the invention.

FIG. 7 is a cross-sectional view of the magnetron of FIG. 4.

FIG. 8 is a plan view of an egg-shaped magnetron.

FIG. 9 is a plan view of a triangularly shaped magnetron.

FIG. 10 is a plan view of a modification of the triangularly shapedmagnetron of FIG. 9, referred to as an arced triangular magnetron.

FIG. 11 is a plan view of the magnets used in the arced triangularmagnetron of FIG. 10.

FIG. 12 is a plan view of two model magnetrons used to calculate areasand peripheral lengths.

FIG. 13 is a graph of the angular dependences of the areas of atriangular and of a circular magnetron.

FIG. 14 is a graph of the angular dependences of the peripheral lengthsof the two types of magnetrons of FIG. 12.

FIG. 15 is a bottom plan view of a magnetron of the invention using barmagnets.

FIG. 16 is a bottom plan view of an alternative to the magnetron of FIG.15.

FIG. 17 is a side view of an idealization of the magnetic field producedwith the described embodiments of the invention.

FIGS. 18 and 19 are a top plan view and a schematic side view of achamber and magnetron arranged for measuring plasma wave generated by amagnetron of the invention.

FIG. 20 is a graph of a typical energy distribution of plasma electrons.

FIG. 21 is a graph showing the effect of RF wafer bias in bottomcoverage in titanium sputtering.

FIG. 22 is a graph of the dependence of chamber pressure upon nitrogenflow illustrating the two modes of deposition obtained in reactivesputtering of titanium nitride with a magnetron of the invention.

FIG. 23 is a graph of the step coverage obtained in the two sputteringmodes for reactive sputtering of titanium nitride with a magnetron ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the invention is a racetrack magnetron 60, illustratedin plan view in FIG. 4. The racetrack magnetron 60 has a centralbar-shaped pole face 62 of one magnetic polarity having opposed parallelmiddle straight sides 64 connected by two rounded ends 66. The central,bar-shaped pole face 62 is surrounded by an outer elongated ring-shapedpole face 68 of the other polarity with a gap 70 of nearly constantwidth separating the bar-shaped and ring-shaped pole faces 62, 68. Theouter pole face 68 of the other magnetic polarity includes opposedparallel middle straight sections 72 connected by two rounded ends 74 ingeneral central symmetry with the inner pole face 62. The middlesections 72 and rounded ends 74 are bands having nearly equal widths.Magnets, to be described shortly, cause the pole faces 62, 68 to haveopposed magnetic polarities. A backing plate, also to be describedshortly, provides both a magnetic yoke between the magnetically opposedpole faces 62, 68 and support for the magnetron structure.

Although the two pole faces 62, 68 are illustrated with specificmagnetic polarities producing magnetic fields extending generallyperpendicularly to the plane of illustration, it is of courseappreciated that the opposite set of magnetic polarities will producethe same general magnetic effects as far as the invention is concerned.The illustrated assembly produces a generally semi-toroidal magneticfield having parallel arcs extending perpendicularly to a closed pathwith a minimal field-free region in the center. There results a closedtunnel of magnetic field lines forming struts of the tunnel.

The pole assembly of FIG. 4 is intended to be continuously rotatedduring sputter deposition at a fairly high rotation rate about arotation axis 78 approximately coincident with the center of the target14 of uniform composition. The rotation axis 78 is located at or nearone prolate end 80 of the outer pole face 68 and with its other prolateend 82 located approximately at the outer radial usable extent of thetarget 14. The asymmetric placement of the rotating magnetron 60 withrespect to the target center provides a small magnetron nonethelessachieving full target coverage. The outer usable periphery of the targetis not easily defined because different magnetron designs use differentportions of the same target. However, it is bounded by the flat area ofthe target and almost always extends to significantly beyond thediameter of the wafer being sputter deposited and is somewhat less thanthe area of the target face. For 200 mm wafers, target faces of 325 mmare typical. A 15% unused target radius may be considered as an upperpractical limit. Racetrack magnetrons are well known in the prior art,but they are generally positioned symmetrically about the center of thetarget. In the described invention, the racetrack is asymmetricallypositioned with its inner end either overlying the target center orterminating at a radial position preferably within 20% and morepreferably within 10% of the target radius from the target center. Theillustrated racetrack extends along a diameter of the target.

As illustrated in the plan view of FIG. 5, two sets of magnets 90, 92are disposed in back of the pole faces 62, 68 to produce the twomagnetic polarities. The combination of the pole faces 62, 68, themagnets 90, 92, and possibly a back magnetic yoke produces two oppositemagnetic poles having areas defined by the pole faces 62, 68. Othermeans may be used to achieved such poles.

The two types of magnets 90, 92 may be of similar construction andcomposition producing an axially extending magnetic flux on eachvertically facing end. If they are of different, magnetic composition,diameter, or length, the flux produced by different magnets may bedifferent. A cross-sectional view of a magnet 90, 92 is shown in FIG. 6.A cylindrical magnetic core 93 extending along an axis is composed of astrongly magnetic material, such as neodymium boron iron (NdBFe).Because such a material is easily oxidized, the core 93 is encapsulatedin a case made of a tubular sidewall 94 and two generally circular caps96 welded together to form an air-tight canister. The caps 96 arecomposed of a soft magnetic material, preferably SS410 stainless steel,and the tubular sidewall 96 is composed of a non-magnetic material,preferably SS304 stainless steel. Each cap 96 includes an axiallyextending pin 97, which engages a corresponding capture hole in one ofthe pole faces 62, 68 or in a magnetic yoke to be shortly described.Thereby, the magnets 90, 92 are fixed in the magnetron. The magneticcore 93 is magnetized along its axial direction, but the two differenttypes of magnets 90, 92 are oriented in the magnetron 60, as illustratedin the cross-sectional view of FIG. 7, so that the magnets 90 of theinner pole 62 are aligned to have their magnetic field extendingvertically in one direction, and the magnets 92 of the outer pole 68 arealigned to have their magnetic field extending vertically in the otherdirection. That is, they have opposed magnetic polarities.

As illustrated in the cross-sectional view of FIG. 7, the magnets 90, 92are arranged closely above (using the orientation of FIG. 1) the polefaces 62, 68 located just above the back of the target 14. A magneticyoke 98 having a generally closed shape generally conforming to theouter periphery of the outer pole face 68 is closely positioned in backof the magnets 90, 92 to magnetically couple the two poles 62, 68. Asmentioned previously, holes in the pole faces 62, 68 and in the yoke 98fix the magnets 90, 92, and unillustrated hardware fix the pole faces62, 68 to the yoke 98.

The inner magnets 90 and inner pole face 62 constitute an inner pole ofone magnetic polarity while the outer magnets 92 and the outer pole face68 constitute a surrounding outer pole of the other magnetic polarity.The magnetic yoke 98 magnetically couples the inner and outer poles andsubstantially confines the magnetic field on the back or top side of themagnetron to the yoke 98. A semi-toroidal magnetic field 100 is therebyproduced, which extends through the non-magnetic target 14 into thevacuum chamber 12 to define the high-density plasma region 38. The field100 extends through the non-magnetic target 14 into the vacuum chamber12 to define the extent of the high-density plasma region 38. Themagnets 90, 92 may be of different magnetic strength. However, it isdesired for reasons to be explained later that the total magnetic fluxproduced by the outer magnets 92 be substantially greater than thatproduced by the inner magnets 90. As illustrated, the magnetron 60extends horizontally from approximately the center of the target 14 tothe edge of the usable area of the target 14. The magnetic yoke 90 andthe two pole faces 62, 68 are preferably plates formed of a softmagnetic material such as SS416 stainless steel.

The inner prolate end 80 of the magnetron 60 is connected to a shaft 104extending along the rotation axis 78 and rotated by a motor 106. Asillustrated, the magnetron 60 extends horizontally from approximatelythe center of the target 14 to the right hand side of the usable area ofthe target 14. Demaray et al. in U.S. Pat. No. 5,252,194 discloseexemplary details of the connections between the motor 106, themagnetron 60, and the vacuum chamber 12. The magnetron assembly 60should include counter-weighting to avoid flexing of the shaft 104.Although the center of rotation 78 is preferably disposed within theinner prolate end 74 of the outer pole face 72, its position may beoptimized to a slightly different position, but one preferably notdeviating more than 20%, more preferably 10%, from the inner prolate end80 as normalized to the prolate length of the magnetron 60. Mostpreferably, the inner end of the outer pole face 68 near the prolate end80 overlies the rotation center 78.

The racetrack configuration of FIG. 4 has the advantage of simplicityand a very small area while still providing full target coverage. Aswill be discussed later, the asymmetric magnetic flux of the two polesis advantageous for low-pressure sputtering and sustainedself-sputtering.

The racetrack configuration of FIG. 4 can be alternatively characterizedas an extremely flattened oval. Other oval shapes are also includedwithin the invention, for example, continuously curved shapes ofcontinuously changing diameter such as elliptical shapes with the majoraxis of the ellipse extending along the radius of the target and withthe minor axis preferably parallel to a rotational circumference.Tabuchi illustrates a symmetric oval magnetron in Laid-open JapanesePatent Application 63-282263. This shape however has the disadvantage ofa complex shape, especially for packing the magnets in the inner pole.

Another oval shape is represented by an egg-shaped magnetron 106,illustrated in plan view in FIG. 8. It has an outer pole face 108 of onemagnetic polarity surrounding an inner pole face 110 of the otherpolarity with a nearly constant gap 122 between them. Both pole faces108, 110 are shaped like the outline of an egg with a major axisextending along the radius of the target. However, an inner end 112 ofthe outer pole face 108 near the rotation axis 78 is sharper than anouter end 114 near the periphery of the target. The egg shape is relatedto an elliptical shape but is asymmetric with respect to the targetradius. Specifically, the minor axis is pushed closer to the targetperiphery than its center. The inner pole face 110 and the gap 122 aresimilarly shaped. Such an egg shape places more of the magnetic fluxcloser to the target periphery so as to improve sputtering uniformity.Such a preferred flux distribution may be characterized with respect tothe half radius of the target extending from its center to its outerusable radius. For improved uniformity, the total magnetic flux locatedoutside the half radius is greater than that located inside the halfradius, for example, by at least a 3:2 ratio, and preferably between 1.8and 2.3. The ratio of magnetic flux outside to inside the target halfradius in this configuration is about 2:1.

A related shape is represented by a triangular magnetron 126,illustrated in plan view in FIG. 9. It has a triangular outer pole face128 of one magnetic polarity surrounding a substantially solid innerpole face 130 of the other magnetic polarity with a gap 132 betweenthem. The triangular shape of the inner pole face 130 with roundedcorners allows hexagonal close packing of the button magnets 90, 92 ofFIG. 6. The outer pole face 128 has three straight sections 134, whichare preferably offset by 60° with respect to each other and areconnected by rounded corners 136. Preferably, the rounded corners 136have smaller lengths than the straight sections 134. One rounded corner136 is located near the rotation center 78 and target center, preferablywithin 20%, more preferably within 10% of the target radius, and mostpreferably with the apex portion of the outer pole face 128 overlyingthe rotation center 78. The triangularly shaped inner pole piece 130 mayinclude a central aperture, but it is preferred that the size of such anaperture be kept small to minimize the size of the central magneticcusp.

A modified triangular shape is represented by an arced triangularmagnetron 140 of FIG. 10. It includes the triangular inner pole face 130surrounded by an arced triangular outer pole face 142 with a gap 144between them and between the magnets of the respective poles and withthe magnetic yoke in back of the gap 144. The outer pole face 142includes two straight sections 146 connected to each other by a roundedapex corner 148 and connected to an arc section 150 by roundedcircumferential corners 152. The apex corner 148 is placed near therotational center 78 and the target center, preferably within 20% andmore preferably within 10% of the target radius. The arc section 150 islocated generally near the circumferential periphery of the target. Itcurvature may be equal to that of the target, that is, be equidistantfrom the center of rotation 78, but other optimized curvatures may bechosen for an arc section concave with respect to the rotational center78. It is located near the target periphery within the chamber,preferably within 25% and more preferably within 15% of the radius tothe periphery. Yokoyama et al. in Laid-open Japanese Patent Application62-89864 discloses the advantage of a plurality of arced triangularmagnetrons arranged symmetrically about the target center. However,plural magnetrons do not provide for a small total area and thus do notachieve a high power density for sputtering. Furthermore, the apices ofthe individual magnetron sections in Yokoyama et al. are locatedrelatively far from the target center, thus producing poor sputteringuniformity except for a large number of sections.

The magnetic field is produced by an arrangement of magnets shown inplan view in FIG. 11. Magnets 160 of a first polarity are disposedadjacent to the inner pole face 130 in an advantageous hexagonallyclose-packed arrangement. Magnets 162 of a second polarity are arrangedadjacent to the arc section 150 of the outer pole face 142 while magnets164 of the second polarity are arranged adjacent to the remainingportions of the outer pole face 142. In some situations, to be describedlater, it is advantageous to place magnets of different intensities atdifferent portions of the outer pole face 142. In one embodiment, thereare 10 magnets in the inner pole and 26 magnets in the outer pole, whichfor magnets of equal strength produces 2.6 more magnetic flux in theouter pole than in the inner pole.

The triangular magnetrons 126, 140 of FIGS. 9 and 10 are illustrated ashaving apex angles θ of 60°, which facilitates promotes hexagonal closepacking of the button magnets, but the apex angle can be changed, inparticular decreased below 60°. However, 60°±15° seems to providesuperior uniformity. The apex angle significantly affects two importantparameters of the magnetron of the invention, the values of its area Aand its perimeter P. Some simple calculations, most easily done for thearced triangular magnetron 140, show the general effects of changing theapex angle θ, as illustrated in plan view in FIG. 12. A simplified ormodel arced triangular magnetron 170 has two straight sides extendingbetween the center and periphery of the target 14 of radius R_(T) andmeeting at an apex coincident with the rotation axis 78 and furtherincludes an arc side conforming to the usable periphery of the target14. The area A of the simplified arced triangular magnetron 170 isθR_(T) ²/2, and its periphery P is R_(T)(2+θ), where θ is measured inradians. Also illustrated in FIG. 12 is a model circular magnetron 172having a radius of R_(T)/2 and having a diameter fixed to the rotationaxis 78. It has an area A of πR_(T) ²/4 and a periphery P of πR_(T).Both magnetrons 170, 172 provide full target coverage. The dependence ofthe arced triangular area A upon the apex angle θ is plotted innormalized units in FIG. 13 by line 174 and that for the circular areaby line 176. Below 90°, the triangular area is smaller. The dependenceof the triangular periphery P is plotted in FIG. 14 by line 178 and thatfor the circular periphery by line 180. Below 65.4°, the arcedtriangular periphery is smaller. Ionization efficiency is increased byminimizing the area, since the target power is concentrated in a smallerarea, and is also increased by minimizing the periphery, since edge lossis generally proportional to the peripheral length. Of course, the areaneeds to be large enough to accommodate the magnets creating themagnetic field. Also, these calculations do not address uniformity. Itis likely that the circular magnetron 170 provides reduced uniformityrelative to the arced triangular magnetron 172.

The ratio of the magnetic flux outside to inside the target half radiusfor the arced triangular magnetron 172 can be approximated by thelengths of the sides 170 in the two regions by (1+θ), which is 1.79 foran apex angle θ of 45°, 2.05 for 60°, 2.31 for 75°, and 2.57 for 90°.

A variation of the arced triangular arrangement of FIGS. 10 and 11decreases the apex angle to, for example, 47°. In addition to thehexagonally close packed inner magnets 160, one or more inner magnetsare linearly arranged from the inner corner of hexagonally closed packedmagnets toward the inner corner of the outer magnets 164. The result isintermediate the racetrack magnetron and the arced triangular magnetron

The experimental work producing the process results presented below hasdemonstrated the advantage of a small magnetron area. If the triangularmagnetron configuration of FIGS. 11 and 12 is adjusted to havesignificantly smaller apex angle θ with a reduced gap between the innerand outer poles, the total magnetic flux produced is limited by thepermeability of the magnets. Therefore, as the apex angle and gap aredecreased, the magnetic field across the gap does not extend so far awayfrom the magnetron. As a result, the high-density plasma extends over anincreasingly shallow height in front of the target. One approach toincrease the effective magnetic flux is to use bar magnets instead ofbutton magnets. The bar magnets have a larger fill factor in the polearea so that for a given total area and a maximum magnetic permeability(per unit area of magnet), a large magnet flux is produced.

A bottom plan view of such a magnetron 190 is illustrated in FIG. 15.Two long side bar magnets 192 and one shorter base bar magnets 194, eachof a same first vertical magnetic polarization are arranged in atriangular shape having an apex angle of, for example, 15°. By a barmagnet is meant a magnet having at least one pair of parallel sidesextending in a direction perpendicular to the direction ofmagnetization. The illustrated magnets 192, 194 have two pairs ofparallel sides so that they are rectangular. However, it is possiblethat one or both ends are shaped, especially the end near the apex. Acircular central magnet 196 of the opposed second vertical magneticpolarization is mostly surrounded by the bar magnets 192, 194. The sidesof the magnets 192, 194, 196 opposite the target are magneticallycoupled by a triangular magnetic yoke 198 although the shape of the yoke198 is not that important as long as the yoke 198 supports the otherwiseseparate and disconnected magnets 192, 194, 196 and magnetically couplesthem. The rotation axis 78 for the yoke 198 and the magnets 192, 194,196 is located near the apex of the triangular shape. The base outermagnet 194 is located near the target periphery just outside of theintended sputtering area of the target. No separate pole face isrequired on the sides of the magnets facing the target since the magnetsprovide a fairly uniform magnetic field. The fill factor for a singleline of circular magnets is π/4=0.79 compared to a bar magnet of equalwidth so that the bar magnet is capable of producing 20% higher totalmagnetic flux.

The illustrated triangular magnetron 190 has an apex angle of 23°. Otherangles may be chosen, but the bar magnets seem particularly applicablewhen the apex angle is between 10° and 35° although apex angles ofbetween 20° and 30° are more realistic. Also, the advantages of the barmagnets are mostly achieved by the side magnets being bar magnets. Theend magnet 194 may be replaced by a magnet or magnetic pole of morecomplex shape. In the original implementation of the bar magnetron 190,the button magnet has a diameter of 0.625 inch (16 mm), and the barmagnets 192, 194, 196 have widths of 1 inch (25 mm), all magnetsproducing the same magnetic field per unit area. In a newer version, thewidth of the bar magnets is being increased to 1.5 inch (38 mm).

In an alternative embodiment of a magnetron 190′ illustrated in thebottom plan view of FIG. 16, a rectangular bar magnet 199 is used as thecentral magnet of the second magnetic polarity. The central magnet canassume a more complex shape, for example, a corresponding but smallertriangular shape or two circular magnets of different diameters so as tobetter fill the interior of the outer magnets and to make the inter-polegap more uniform. However, the illustrated configurations have beenshown to be quite effective.

It is understood that the shapes described above refer to pole faceshaving band-like widths of area not significantly larger than the buttonmagnets being used. The widths, particularly of the outer pole face, canbe increased, perhaps even non-uniformly, but the additional width is ofless effectiveness in generating the desired high magnetic field.

The shapes presented above have all been symmetric about the targetradius. However, the magnetron of the invention includes asymmetricshapes, for example one radially extending side being in the form of theracetrack of FIG. 4 and the other side being oval, e.g., the egg shapeof FIG. 7, or one radially extending side being oval or straight and theother side having a triangular apex between the center and periphery ofthe target.

All the magnetrons described above have asymmetric areas for the innerand outer poles and, assuming similar packing of similar button magnets90, 92 or bar magnets of similar magnetic intensities, they produceasymmetric magnetic flux. In particular, the total or integratedmagnetic flux ∫B·dS produced by the inner pole 200, illustratedschematically in FIG. 17, is much less than that produced by thesurrounding outer pole 202, for example, by at least a factor of 1.5 andpreferably 2. All the magnetrons are also characterized as having acompact inner pole 200 surrounded by the outer pole 202. The result is amagnetic field distribution which is very strong in the reactorprocessing area 204 adjacent to the gap 206 between the poles 200, 202,but which also extends far into the processing area 204 as the magneticfield lines of the outer pole 192 close back to the magnetic yoke 198.The substantial fraction of magnetic field extending vertically from thetarget deep into the processing area 204 offers many advantages. Becausethe light electrons orbit around magnetic field lines, the extendedmagnetic field traps electrons and thus helps to support ahigher-density plasma deep into the processing area 204. By the sameinteraction, the magnetic field extending close and parallel to thegrounded chamber shield reduces electron loss to the shield, alsoincreasing the density of the plasma. As a result, the plasma can besupported at lower pressure or even be self-sustained. The magneticfield also partially traps heavier positive particles and thus guidesionized sputtered particles towards the wafer.

The inventive magnet also achieves a relatively high magnetic field.However, magnetic field intensity of itself is insufficient. Someconventional magnetrons, such as Demaray et al. disclose in theaforecited patent, use a line of horseshoe magnets arranged in akidney-shaped linear path with only a small gap between the poles of thehorseshoes. As a result, a relatively high magnetic field intensity canbe achieved in the area at the periphery of the kidney shape. However,the linear shape of the high magnetic field surrounds an area ofsubstantially no magnetic field. As a result, electrons can escape tonot only the exterior but also the interior of the high-field region. Incontrast, the inner pole of the triangular magnetron of the inventionproduces a magnetic cusp of minimal area. If electrons are lost from themagnetic field on one side of the inner pole, they are likely to becaptured on the other side, thus increasing the plasma density for agiven power level. Furthermore, the inner pole includes a singlemagnetizable pole face producing a generally uniform magnetic flux. Ifmultiple inner poles faces were used for multiple inner magnets,magnetic field lines would extend to between the inner magnets.

A further advantage of the inventive design is that one pole is formedin a closed line and surrounds the other pole. It would be possible toform a very small linearly extending magnetron with high magnetic fieldintensity by arranging horseshoe magnets or the like in an open endedline with the two sets of poles being closely spaced. However, theelectrons could then easily escape from the open ends and decrease thedensity of the plasma.

It is believed that the beneficial results of the invention are achievedin large part because the oval magnetrons and magnetrons of relatedshapes produce a higher plasma ionization density without requiringexcessive power. Nonetheless, full target coverage is achieved. In oneaspect, the inventive magnetron has a relatively small area, but has ashape that allows full target coverage without radial scanning. Thetriangular magnetron 160 of FIG. 10 with an apex angle of 60° has anarea of ⅙ (0.166) of the usable target area. In contrast, if thecircular magnetron 162 were used, which similarly extends from thetarget center to the periphery, the magnetron area is ¼ (0.25) of thetarget area. As a result, the power density is less for a given powersupply powering a larger circular magnetron. The target overlaypercentage is even higher for the Tepman magnet of FIG. 3.

The combination of small area and full coverage is achieved by an outermagnetron shape extending from the target center to its usable periphery(±15%) and having a transverse dimension at half the target radius ofless substantially less than the target radius, that is, prolate alongthe target radius. The transverse dimension should be measuredcircumferentially along the rotation path.

The uniformity is enhanced by an oval shape that is transversely wider,with respect to the target radius, at its outer end near the targetperiphery than at its inner end near the center of rotation. That is,the minor axis is displaced towards the target circumference.

The small area of the magnetron, but nonetheless providing full targetcoverage, allows a very high power density to be applied to the targetwith a reasonably sized power supply. The small area, unlike the Tepmandesign, has no substantial field-free region included in its interior.Some of the examples below use an 18 kW power source. For a 200 mmwafer, the magnetron extends out to a usable target diameter of about300 mm. The effective area of the arced triangular magnetron is aboutone-sixth of the area associated with this larger diameter, that is,about 117 cm². Thus, the average power density of the area beingsputtered at any given location of the magnetron is about 150 W/cm².Such a high power density achieved without inductive coils can support aplasma at lower argon pressure or permit sustained self-sputtering forselected metals such as copper. Even with 300 mm wafers, a 27 kW powersupply in conjunction with the small magnetron of the invention scaledto the larger dimension will produce a target power density of 103W/cm². As shown below, a power density of 76 W/cm² is sufficient forsustained self-sputtering of copper.

Wave-Beam Interaction

The magnetrons of the type described above produce an unexpectedly highmetal ionization fraction, on the order of 10 to 20%. While this isbelow the 50 to 70% metal ionization fraction experienced in inductivelycoupled IMP reactors, it is still substantially higher than the lessthan 5% metal ionization fraction usually experienced in DC magnetronreactors. Experiments have shown that the above described magnetrons canexcite several plasma waves, and it is believed that these wavesincrease the energy of the plasma electrons and the increased electronenergy significantly increases the ionization of the sputtered metalatoms. It is known that relatively small increases in the electronenergy (temperature) significantly increases plasma densities.

A series of experiments were performed using a triangular magnetron 210illustrated in the plan view of FIG. 18 and the side view of FIG. 19including a generally triangular outer pole 212 surrounding an innerpole 214 of the opposite magnetic polarity. The magnetron 210 is placedbehind a 1.2 cm-thick planar target 14 of titanium sealed to theotherwise conventional sputter reactor of FIG. 1. However, the magnetron210 is not rotated during the tests, and various probes 218 are insertedfrom the below with the probe tip located about 1 cm below the target 14at a position between the magnetron poles 212, 214 at about two-thirdsof the target radius. Typical chamber operating conditions used duringthe tests are an argon gas pressure of 1.6 milliTorr and 2 kW of DCtarget power producing a target voltage of 455VDC.

A spectrum analyzer having a floating cylindrical Langmuir probe as theprobe reveals a double-peaked feature at about 240 MHz and 262 MHz and abroad feature at about 22 MHz. It is our interpretation, although theinvention is not limited to our understanding of the invention, that the262 MHz peak is a lower-hybrid peak ω_(LH) and that the other two peaksare produced by a non-linear parametric conversion in which the 22 MHzpeak has an energy ω_(B) and the 240 MHz sideband peak has an energyω_(LH)-ω_(B). In a parametric conversion, the wave vector is alsoconserved so that wave vectors for the 22 and 240 MHz peak should berelated as k_(B) and k_(LH)-k_(B).

A further discussion of this interpretation requires some definitions. Aplasma has two plasma frequencies associated with the electrons and theions. In each case, the plasma frequency ω_(P) can be expressed as${\omega_{P}^{2} = \frac{4e^{2}n}{m}},$where e is the charge which is of unit value for both electrons and mostplasma ions, n_(P) is the plasma density, and m is the mass of theelectron or ion. The plasma also has two cyclotron frequencies ω_(C),which can be expressed as ${\omega_{C} = \frac{eB}{mc}},$where B is the magnetic field and c is the speed of light. We estimatethat the electron plasma frequency is about 3 GHz; the electroncyclotron frequency, about 1 GHz; and the ion plasma frequency, about 11MHz.

Matsuoka et al. have disclosed observing a plasma wave in “Dense plasmaproduction and film deposition by new high-rate sputtering using anelectric mirror,” Journal of Vacuum Science and Technology A, vol. 7,no. 4, July/August 1989, pp. 2652-57. However, they attributed theprimary plasma wave to the upper hybrid mode ω_(UH), which can berepresented byω_(UH)={square root}{square root over (ω_(P,e) ²+ω_(C,e) ².)}This would be too high a frequency to match the observed spectrum.Instead, we believe the 262 MHz peak is associated with the lower hybridmode ω_(LH) which is defined as$\omega_{LH} = {\frac{\omega_{P,e}}{\sqrt{1 + \frac{\omega_{P,e}^{2}}{\omega_{C,e}^{2}}}}.}$Lower hybrid modes can exist with frequencies in the range ofω_(C,i)<ω<ω_(C,e), ω_(P,e).

We believe that the peak at 22 MHz is associated with a lower hybrid ionquasi mode that is associated with the plasma being over driven with thelarge amounts of power being applied to it.

The plasma waves at 240 MHz and 262 MHz, whatever their source, do notprovide much heating of the electrons. As illustrated in the graph ofFIG. 20, plasma electrons have a generally maxwellian energydistribution 220 with an average energy <E> defining an electrontemperature T_(e). Electron temperatures in practical sputtering plasmashave been reported in the range of 3 eV to over 20 eV. The energydistribution 220 has a high sub-peak 222 associated with the secondaryelectrons crossing the plasma sheath with an energy generally associatedwith the target voltage VT, which for our experiments is 455V.

An advantage of the parametric power conversion believed responsible forgenerating the 22 MHz mode is that the higher-frequency modes, such asthe 240 MHz one, are more typical in plasma reactors, but its power isconverted to a lower-frequency mode at less than 20% of the originalfrequency which is more suitable to interact with the thermalized bulkelectrons.

The wave vectors k of the upper two peaks were measured using a digitaloscilloscope having two probes 208 separated by Δx=0.6 cm in therespective r, θ, z directions illustrated in FIGS. 18 and 19. The wavevector component k_(x) is calculated to be Δφ/Δx, where Δφ is themeasured phase difference between the probes. The wave vector k for the262 MHz peak can be represent in units of cm⁻¹ as{right arrow over (k)}=3.9{circumflex over (r)}+6.3{circumflex over(θ)}+0.5{circumflex over (z)},which has a magnitude k of about 7.4 cm⁻¹. The difference in wavevectors between the 240 MHz and the 262 MHz peak has been measured tohave a magnitude |Δk| of approximately 2 cm⁻¹. The error bars on most ofthese measured values are large, but no more than 50%.

The magnetic field vector B has been measured at a point between theprobes can be expressed in units of gauss as{right arrow over (B)}=150{circumflex over (r)}+450{circumflex over(θ)}+35{circumflex over (z)},which has a magnitude of 475 gauss. The angle between the wave vector kand the magnetic field B is equal to the ratio between the perpendicularand parallel wave vectors k⊥/k∥, which is measured to be in the range of0.5 to 0.75.

The phase velocity v_(p) for a wave is given by$v_{p} = {\frac{\omega}{|k|}.}$For the 262 MHz peak, the phase velocity is thus calculated from themeasured wave vector as 2×10⁸ cm/s based on the above measurement of itswave vector. This is to be compared with a velocity of 1×10⁹ cm⁻¹ forthe 455 eV injected secondary electrons. That is, the freshly injectedsecondary electrons could easily drive the measured 262 MHz plasma wave.However, the phase velocity of the 262 MHz peak is entirely too high toeffectively interact with the bulk of the thermalized electrons.

The wave vector for the 22 MHz radiation could not be adequatelymeasured directly. However, assuming a parametric process, the wavevector difference between the 262 MHz and the 240 MHz modes (measured as2 cm⁻¹) should equal the wave vector of the 22 MHz mode. If this istrue, the phase velocity of the 22 MHz mode is approximately 6×10⁷ cm/s,which corresponds to a 10 eV electron. That is, the 22 MHz mode is wellmatched to couple energy into the thermalized plasma electrons, therebyincreasing the average electron energy. We believe the coupling from theplasma wave to the electrons is through Landau damping.

The conditions permitting the launching of the lower hybrid mode and itsparametric conversion to another mode capable of coupling to thethermalized electrons depend greatly on the magnetic configuration andstrength associated with the magnetrons. The magnetrons and planartarget described for this invention appear to satisfy the conditions.Other magnetrons have been tested with planar targets, but no plasmawaves are observed. Apparently, the electron mirror configuration of thecomplexly shaped target of Matsuoka et al. fails to launch the lowerhybrid mode, and they fail to report any wave lower than about 100 MHz.In view of our experience and the apparent phase velocity of the 22 MHzmode, it seems necessary that a plasma mode be excited between 5 and 75MHz, preferably between 10 and 50 MHz, in order to pump the 1 to 20 eVplasma electrons. The launching of any plasma waves seems to depend upona magnetic field projecting far away from the target. Matsuoka et al.accomplish this by a complex hollow cathode design. The presentinvention accomplishes this by the unbalanced magnetic field strengthsof the two poles of the magnetron, which produces a vertical magneticfield far away from the target, as well as by driving the reactor at ahigh power level.

Another condition for launching a plasma wave is that the beam densityof the secondary electrons emitted from the target needs to exceed athreshold. That is, the power density applied to the target must behigh. The inventive magnetron reduces the magnetron area and henceallows an increased power density achievable by a given power supplywhile still maintaining sputtering uniformity. Nonetheless, high targetpower is required for a commercially sized sputter readctor.

Processes

A racetrack magnetron of FIGS. 4 and 5 was tested with coppersputtering. In one configuration, six magnets 90 are placed behind thecenter pole face 62, twenty-five magnets of the same strength butopposite polarity are arranged behind and around the outer pole face 68,and the spacing between the 33 cm target and the 200 mm wafer is 190 mm.This configuration produces a deposition uniformity of ±18%. In a secondconfiguration, the magnets have different strengths, the stronger onesproducing 30% more magnetic flux. Six strong magnets are placed behindthe center pole face, and 25 weaker magnets are placed around the outerpole face. Despite the stronger inner magnets, the total magnetic fluxproduced by the outer magnets is greater than that produced by the innerones. The second configuration produces an improved depositionuniformity of 8.9%. The second configuration also produces superior holefilling into a 0.5 μm-wide, 2 μm-deep via hole. For 265 nm of blanketcopper, the bottom coverage is between 10 and 15%, and the sidewallcoverage is about 2.8%. The deep hole filling is promoted by the smallarea of the racetrack magnetron producing a higher ionization density.In a third configuration, strong magnets replace some of the weakermagnets near the ends of the outer pole. This produces a somewhat betteruniformity.

An arced triangular magnetron of FIGS. 10 and 11 was tested in a seriesof experiments with different sputtering composition. For almost all theexperiments, the target was spaced between 190 and 200 mm from the waferand the target had a diameter of 330 mm for a 200 mm wafer.

Copper

For copper sputtering, uniformity is improved by using ten strongmagnets 160 in the inner pole, strong magnets 162 along the arc portion150 of the outer pole, and weaker magnets 164 for the remainder of theouter pole. The stronger magnets have a diameter 30% larger than thediameter of the weaker magnets, but are otherwise of similar compositionand structure, thereby creating an integrated magnetic flux that is 70%larger.

Sustained self-sputtering of copper is achieved, after striking theplasma in an argon ambient, with 9 kW of DC power applied to the targethaving a usable diameter of about 30 cm, which results in a powerdensity of 76 W/cm² with the arced triangular magnetron. However, it isconsidered desirable to operate with 18 kW of DC power and with aminimal argon pressure of about 0.1 milliTorr arising at least in partfrom leakage of the gas providing backside cooling of the wafer to theliquid-chilled pedestal. The increased background pressure of 0.1 to 0.3milliTorr enhances effective wafer cooling without significant increasein the scattering and deionization of the sputtered ions. Theserelatively low DC powers are important in view of the ongoingdevelopment of equipment for 300 mm wafers, for which these numbersscale to 20 kW and 40 kW. A power supply of greater than 40 kW isconsidered expensive if not infeasible.

One application of ionized copper sputtering is to deposit a thinconformal seed layer of copper in a deep and narrow via hole.Thereafter, electro or electroless plating can be used to quickly andeconomically fill the remainder of the hole with copper.

In one experiment, a via hole having a top width of 0.30 μm andextending through 1.2 μm of silica was first coated with a Ta/TaNbarrier layer. Using the arced triangular magnetron, copper wasdeposited over the barrier layer at 18 kW of target power and a pressureof 0.2 milliTorr. The deposition was carried out to a blanket thicknessof about 0.15 μm. The sides of the via hole was smoothly covered. Theexperiments show that the sidewall thickness of the copper is about 7 nmon one side and 11.4 nm on the other side (5% and 8%) for a via locatedat the wafer edge. The bottom coverage is about 24 nm (16%). Sidewallsymmetry is improved for a via hole at the wafer center. The smoothnesspromotes the use of the deposited layer as a seed layer and as anelectrode for subsequent electroplating of copper. The relatively goodsymmetry between the two sidewalls relieves the problem in the prior artof apparently moving photolithographic indicia.

Aluminum

Using the arced triangular magnetron, sputtering of an aluminum targetwas achieved at both 12 kW and 18 kW of applied power with a minimumpressure of about 0.1 milliTorr, a significant improvement. For aluminumsputtering, sidewall coverage and particularly bottom coverage issignificantly improved. The better uniformity is also believed to berelated in part to the increased ionization fraction since theself-biased pedestal supporting the wafer attracts the ionized sputteredparticles across its entire area. It is estimated that the magnetron ofthe invention increases the ionization fraction from 2% to at least 20%and probably 25%.

The arced triangular magnetron was compared under similar operatingconditions to the operation of a conventional magnetron resembling theTepman magnetron of FIG. 3. The comparative results are summarized inTABLE 1 for the sputtering of aluminum. TABLE 1 Triangle Conv. Bottom28.5% 8.0% Coverage Sidewall 8.0% 5.7% Coverage Uniformity 4.6% 7.8%(190 mm) Uniformity 3.0% 6.0% (290 mm) Minimum 0.1 0.35 Pressure(milliTorr)

The coverage results were obtained for via holes having a width of 0.25μm and a depth of 1.2 μm, that is, an aspect ratio of about 5. Thebottom coverage is significantly improved with the inventive triangularmagnetron compared to the conventional magnetron. The sidewall coverageis also increased, and further the coverage is smooth and uniform fromtop to bottom. These two characteristics promote the use of thedeposited metal layer as a seed layer for a subsequent deposition step.This is particularly important for copper in which the second depositionis performed by a different process such as electroplating. Theincreased bottom and sidewall coverages are believed to be due to thehigher ionization fraction of sputtered aluminum atoms achieved with theinventive triangular magnetron. This ionization fraction is believed tobe 25% or greater. The uniformity of blanket (planar) deposition wasdetermined both for a separation of 190 mm between the target and thewafer and, in a long-throw implementation, for a separation of 290 mm.The inventive triangular magnetron produces better uniformity,especially for long throw. The better uniformity is also believed to berelated to the increased ionization fraction since the self-biasedpedestal supporting the wafer attracts the ionized sputtered particlesacross its entire area. Similarly, the inventive triangular magnetronproduces less asymmetry between the coverages of the two opposedsidewalls. The increased ionization density is due in part to therelatively small inner yoke having an area substantially less than thatof the outer yoke. As a result, electrons lost from one side of theinner yoke are likely to be captured by the other side.

The bar magnetron of FIG. 15 has been demonstrated for sputteringcopper. It shows improved bottom coverage and sidewall coverage near thebottom. Overall, it produces a seed layer of reduced thickness thatstill allows effective hole filling of copper by electrochemicalplating.

All of these advantages are obtainable in a conventional capacitivelycoupled DC sputter reactor using a magnetron of fairly simple design. Ofcourse, the magnetron of the invention can also be advantageously usedin other types of sputter reactors, such as an HDP reactor relying uponinductively coupled RF power.

Titanium

The arced triangular magnetron was also used to sputter titanium.Titanium, sometimes in conjunction with titanium nitride, is useful inaluminum metallization for providing a silicided contact to silicon atthe bottom of a contact hole and to act as wetting layer and inconjunction with a titanium nitride layer as a barrier both to thesilicon in a contact hole and between the aluminum and the silicadielectric on the via or contact sidewalls. Conformal and relativelythick coatings are thus required.

A series of experiments were performed using a titanium target with 18kW of DC target power and with only six magnets 160 in the inner pole.At a chamber pressure of 0.35 milliTorr, the bottom coverage anduniformity are observed to be good.

The titanium experiments were continued to measure bottom coverage as afunction of the aspect ratio (AR) of the via hole being coated. With nowafer bias applied and the pedestal heater 18 left electricallyfloating, the 18 kW of target power nonetheless self-biases the targetto about 30 to 45V. The bottom coverage under these conditions is shownby line 230 in the graph of FIG. 21. The bottom coverage decreases forholes of higher aspect ratios but is still an acceptable 20% at AR=6.

In a continuation of these experiments, an RF power source 232,illustrated in FIG. 1, was connected to the heater pedestal 18 through acapacitive coupling circuit 234. It is known that such an RF fieldapplied to the wafer adjacent to a plasma creates a DC self-bias. When100 W of 400 kHz power is applied with a chamber pressure of 0.3milliTorr, the bottom coverage is significantly increased, as shown byline 236 in the graph of FIG. 21. However, when the bias power isincreased to 250 W, resputtering and faceting of the top corners of thevia hole becomes a problem. The bottom coverage results for 250 W biasare shown by line 238. For aspect ratios above 4.5, the bottom coveragewith 250 W of wafer bias is generally worse than for 100 W of wafer biasso bias powers should be kept below 250 W for lower bias frequencies of2 MHz or less. These powers should be normalized to a 200 mm circularreference wafer. Other sizes of wafers, such as a 300 mm wafer can beused, and these wafers may not be completely circularly because ofindexing flats or notches. However, the same effects are expected whenthe power levels quoted above are referenced to a 200 mm circularreference wafer and then scaled according to the differing area of asubstantially circular working wafer.

The experiments were continued for holes with aspect ratios of 4.5 using300 W of RF wafer bias at a frequency of 13.56 MHz. At a pressure of 0.7milliTorr, the blanket deposition rate is 128 nm/min, and the bottomcoverage varies between 31% and 52%. At a pressure of 1.4 milliTorr, thedeposition rate is 142 nm/min, and the bottom coverage varies between42% and 62%. At the higher pressure, the sidewall coverage variesbetween 10.4% and 11.5%, and no appreciable sidewall asymmetry isobserved. Contrary to expectations, pressures above 0.7 milliTorrproduce higher titanium deposition rates and better bottom coverage. Thehigher bias frequency permits a higher bias power to be applied.

Titanium Nitride

The magnetron of the invention can also be used for reactive sputtering,such as for TiN, in which nitrogen is additionally admitted into thechamber to react with the sputtered metal, for example, with titanium toproduce TiN or with tantalum to produce TaN. Reactive sputteringpresents a more complex and varied chemistry. Reactive sputtering toproduce TiN is known to operate in two modes, metallic mode and poisonmode. Metallic mode produces a high-density, gold-colored film on thewafer. Poison mode, which is often associated with a high nitrogen flow,produces a purple/brown film which advantageously has low stress.However, the poison-mode film has many grain boundaries, and filmdefects severely reduce chip yield. Furthermore, the deposition rate inpoison mode is typically only one-quarter of the rate in metallic mode.It is generally believed that in poison mode the nitrogen reacts withthe target to form a TiN surface on the Ti target while in metallic modethe target surface remains clean and TiN forms only on the wafer.

The arced triangular magnetron was tested for reactive sputtering oftitanium nitride in the same chamber used for sputter depositingtitanium.

The initialization conditions for sputter depositing titanium nitrideare found to be very important to obtain operation in the metallic mode.In a series of initial experiments, argon alone is first admitted to thechamber. After the plasma is struck at an argon pressure of about 0.5milliTorr, the argon flow is reduced to 5 sccm producing a pressure of0.3 milliTorr. When the nitrogen flow is then step wise ramped up to 100sccm and then is gradually reduced, the dependence of the chamberpressure upon the flow assumes a hysteretic form illustrated in FIG. 18.Between about 50 and 70 sccm of nitrogen, intermediate ramp-up pressures240 are below corresponding intermediate ramp-down pressures 242. Atlower pressures 244 and at higher pressures 246, there is no significantseparation between ramp up and ramp down. It is believed that the lowerpressures 244 and intermediate ramp-up pressures 240 cause sputtering inmetallic mode while higher pressures 246 and intermediate ramp-downpressures 242 cause sputtering in poison mode.

These results show that, for higher operational deposition rates in thegenerally preferred metallic mode, it is important to not exceed theintermediate ramp-up pressures 240, that is, not to exceed the maximummetallic-mode flow, which in these experiments is 70 sccm or slightlyhigher but definitely below 80 sccm. The argon and nitrogen can besimultaneously and quickly turned on, but preferably the DC power isalso quickly turned on.

There are some applications, however, where operation in poison mode ispreferred. This can be achieved by first going to the higher pressures246 and then decreasing to the ramp-down intermediate pressures 242.Alternatively, poison mode can be achieved by immediately turning on thedesired gas flow, but only gradually turning on the DC sputtering powersupply at a rate of no more than 5 kW/s.

Titanium nitride was sputtered into high aspect-ratio via holes in bothmetallic and poison modes at a N₂ flow of 50 sccm and an Ar flow of 5sccm after the plasma had been struck in argon. These flows produce apressure of 1.7 milliTorr in metallic mode and 2.1 milliTorr in poisonmode. The deposition rates are 100 nm/min in metallic mode and 30 nm/minin poison mode. On one hand, the TiN film stress is higher when it isdeposited in metallic mode, but on the other hand poison mode suffersfrom overhang and undulatory sidewall thicknesses near the top of thevia hole. A series of experiments deposited TiN into via holes ofdiffering aspect ratios. The resulting measured bottom coverage,illustrated in the graph of FIG. 23, shows in line 250 that bottomcoverage in metallic mode remains relatively high even with an viaaspect ratio of 5 while in line 252 the step coverage in poison mode isalways lower and drops dramatically for aspect ratios of four andhigher. However, when additionally the wafer is biased, step coverage ofTiN deposited in the poison mode is acceptable.

The success of depositing TiN in the same chamber used for depositing Tidemonstrates that the Ti/TiN barrier can be deposited according to theinvention in one continuous operation.

Integrated Tungsten Plug Process

Two series of tests were performed to demonstrate an integrated processcombining the Ti/TiN barrier deposited with the arced magnetron of theinvention and a tungsten plug deposited by chemical vapor deposition(CVD) into the barrier-coated hole. This combination has presented someproblems in the past because tungsten CVD typically uses tungstenhexafluoride (WF₆) as the gaseous precursor. WF₆ tends to attack Ti andto result in structures formed in the W plug resembling volcanoes, whichproduces voids in the plug.

In the first series of tests, the barrier layer consisted of 30 nm of Ticovered by 30 nm of TiN deposited with the arced magnetron of theinvention in an otherwise conventional non-inductive sputter reactor.Following the Ti/TiN deposition, the chip was subjected to rapid thermalprocessing (RTP) in which intense radiant lamps quickly heat the wafersurface for a short period. In the second series of tests, the barrierlayer consisted of 30 nm of Ti covered by 10 nm of TiN deposited as inthe first series. However, in the second test, before the Ti/TiNdeposition the wafer was subjected to a plasma preclean, but there wasno RTP afterwards. In either case, tungsten was then CVD deposited overthe Ti/TiN.

These experiments show that neither process produces volcanoes.Furthermore, thickness and resistivity of the TiN show good uniformity.The TiN resistivity is measured to be less than 45 μΩ-cm. Bottomcoverage of 20% for the TiN/Ti bilayer is observed in holes havingaspect ratios of 5:1 without the use of wafer biasing. Wafer biasingproduces the same bottom coverage in holes having aspect ratios of 10:1.Thus, the Ti/TiN process performed with the magnetron of the inventioncan be successfully integrated into a tungsten plug process. Theinventive magnetron can also be used to sputter deposit other materials,for example, W, using a tungsten target, or TaN, using a tantalum targetand nitrogen gas in the plasma. Reactive sputtering of WN is alsocontemplated.

The magnetron of the invention is thus efficient in producing a highionization fraction because of the high-density plasma it can createwithout excessive power being required. Nonetheless, its full coverageallows for uniform deposition and full target utilization. Itssputtering uniformity is good. Nonetheless, no complex mechanisms arerequired.

The effectiveness of the magnetron of the invention in providinghigh-performance full-coverage sputtering is based on three interrelatedsynergetic effects. The magnetron has a small magnetic area. Thereby,the average magnetic field can be made high, and the plasma lossesreduced. The small magnetron also allows a high average power density tobe applied to the area of the target beneath the magnetron. That is,although the electrical power applied to the target as a whole isrelatively modest, the electrical power density and resulting plasmadensity in the area actually being sputtered at any instant is high. Theasymmetry of the inner and outer magnetic poles of the magnetronproduces portions of the magnetic field extending vertically surroundingthe periphery of the magnetron and extending far into the chamber. Thismagnetic field distribution reduces plasma losses and guides ionizedsputtered particles to the substrate. All of these advantages areenjoyed in a magnetron providing full coverage sputtering of the targetwith only circumferential scanning, and in a magnetron that can beoptimally shaped to produce uniform target sputtering and uniformsubstrate deposition.

Such a small, high-field magnet enables sustained self-sputtering withrelatively modest target power and also enables sputtering of materialssuch as aluminum and titanium at reduced pressures below 0.5 milliTorr,preferably below 0.2 milliTorr, and even at 0.1 milliTorr. At thesepressures, deep hole filling can be facilitated by the reducedscattering of sputtered particles, whether neutral or ionized, and bythe reduced neutralization of ionized particles. However, at least fortitanium, it has been found that with the use of the magnetron of theinvention, deposition rate and bottom coverage are improved with workinggas pressures above 0.7 milliTorr. The high-field magnet furtherpromotes a high ionization fraction, which can be drawn into a deep,narrow hole by biasing of the wafer within proper ranges.

1. A method of sputtering material from a target comprising a metal ontoa working substrate supported on a pedestal in a system including amagnetron disposed on a side of said target opposite said pedestal alonga central axis of a vacuum chamber containing said pedestal andincluding an outer pole having a first magnetic polarity and a firsttotal magnetic flux and an inner pole surrounded by said outer pole andhaving a second magnetic polarity opposite said first magnetic polarityand a second total magnetic flux which is larger than said first totalmagnetic flux by a factor of at least 1.5, said method comprising thesteps of: rotating said magnetron about said central axis; admitting aworking gas into said vacuum chamber; and applying DC power to saidtarget to excite said working gas into a plasma to thereby sputter saidmetal of said target onto said substrate.
 2. The method of claim 1,wherein said metal is tantalum.
 3. The method of claim 1, wherein saidmetal is titanium.
 4. The method of claim 1, wherein said metal istungsten.
 5. The method of claim 1, further comprising admitting gaseousnitrogen into said vacuum chamber, wherein a nitride of said metal isformed on said substrate.
 6. The method of claim 5, wherein said metalis tantalum.
 7. The method of claim 5, wherein said metal is titanium.8. The method of claim 5, wherein said metal is tungsten.
 9. The methodof claim 1, wherein said factor is at least 2.0.
 10. The method of claim1, wherein an area within a periphery of said magnetron is no more than⅙ of a usable area of said target.
 11. The method of claim 1, furthercomprising RF biasing said pedestal.
 12. A tantalum sputtering methodperformed in a plasma sputter reactor having a tantalum target disposedon one side of a vacuum chamber and arranged about a central axis,comprising the steps of: supporting a substrate to be sputter coated ona pedestal electrode arranged opposite said target along said centralaxis; rotating a magnetron disposed on a side of said target oppositesaid pedestal about said central axis, said magnetron including an innerpole of a first magnetic polarity and having a first total magnetic fluxand an outer pole of a second magnetic polarity opposite said firstmagnetic polarity, having a second total magnetic flux greater than saidfirst total magnetic flux by a factor of at least 1.5, and surroundingsaid first magnetic pole; admitting argon into said vacuum chamber;applying negative DC power to said target to excite said argon into aplasma to sputter said target; and RF biasing said pedestal electrode toinduce a negative DC self-bias thereupon.
 13. The method of claim 12,wherein said factor is at least 2.0.
 14. The method of claim 12, whereinan area within a periphery of said magnetron is no more than ⅙ of anarea of said target.
 15. The method of claim 12, further comprisingadmitting nitrogen into said vacuum chamber, whereby tantalum nitride isdeposited on said substrate.
 16. A tantalum plasma sputter reactor,comprising: a vacuum chamber; a tantalum target disposed on a side ofsaid vacuum chamber; a pedestal electrode disposed in said vacuumchamber in opposition to said target for supporting a substrate to besputter coated; and a magnetron rotatable about said central axis andincluding an inner magnetic pole having a first magnetic polarity and afirst total magnetic flux and an outer magnetic pole surrounding saidinner magnetic pole and having a second magnetic polarity opposite saidfirst magnetic polarity and a second total magnetic flux greater thansaid first total magnetic flux by a ratio of at least 1.5.
 17. Thereactor of claim 16, wherein said ratio is at least 2.0.
 18. The reactorof claim 16, wherein an area within a periphery of said magnetron is nomore than ⅙ of an area of said target.
 19. The reactor of claim 16,further comprising an RF power supply connected to said pedestalelectrode.